System for the introduction of coolant into open-circuit cooled turbine buckets

ABSTRACT

An improved coolant distribution system is described for the introduction of liquid coolant into the open-circuit cooling channels of a turbine bucket. Liquid coolant is transported by centrifugal force from the gutter to a pool located at the underside of the platform, the liquid entering the pool beneath the surface thereof. At least one weir is formed in the pool periphery for controlled distribution of the coolant from the pool to cooling channels in one or more turbine buckets.

United States Patent 1191 Moore 15 1 SYSTEM FOR, THE mrnonucnou 0FCOOLANT INTO OPEN-CIRCUIT COOLED TURBINE BUCKETS [75] Inventor:

[73] Assignee:

John Moore, Schenectady, NY.

General Electric Company, 1 Schenectady, NY.

1451 Apr. 16, 1974 Kydd 416/97 FOREIGN PATENTS OR APPLICATIONS 1,801,4754/1970 Germany 416/96 Primary Examiner-Everette A. Powell Attorney,Agent, or Firm-Leo I. MaLossi; Joseph T; Cohen; Jerome C. Squillaro [57] ABSTRACT An improved coolant distribution system is described for theintroduction of liquid coolant into the opencircuit cooling channels ofa turbine buc'ket. Liquid coolant is transported by centrifugal forcefrom the gutter to a pool located at the underside of the platform, theliquid entering the pool beneath the surface thereof. At least one weiris formed in the pool periphcry for controlled distribution of thecoolant from the pool to cooling channels in one or more turbinebuckets.

8 Claims, 4 Drawing Figures WENTEUAPR 16 19m SHEET 2 0f 3 SYSTEM FOR THEINTRODUCTION OF COOLANT INTO OPEN-CIRCUIT COOLED TURBINE BUCKETS IBACKGROUND OF THE INVENTION Structural arrangements for the open-circuitliquid cooling of gas turbine buckets are shown in U. S. Pat. Nos.3,446,481 Kydd and 3,446,482 Kydd. An arrangement for the metering ofliquid coolant to such buckets is shown in U. S. Pat. No. 3,658,439Kydd. These patents are incorporated by reference.

Open-circuit liquid cooling capability is particularly important becauseit makes feasible increasing the turbine inlet temperature to anoperating range of from 2,500 F to at least 3,500 F thereby obtaining anincrease in power output ranging from about 100 to 200 percent and anincrease in thermal efficiency ranging to as high as 50 percent. Suchopen-circuit liquid cooled turbine structures are referred to as ultrahigh temperature gas turbines.

Liquid coolant metering is complicated by the extremely high buckettip'speeds employed resulting in centrifugal fields of the order of250,000 G. Ideally, liquid fiow to each coolant passage would becorrelated with the quantity of heat it is desired to transfer to thatgiven passage. Since liquid coolant is distributed to the coolantpassages by flow over a weir structure, improvements that couldpredictably and controllably distribute weir flow at least as anapproximate function of FIG. 1 shows a unified bucket/rotor diskrimconstruction embodying the means of the instant invention for conductingliquid to the pool/weir structure and power augmentation means of thegeneral type disclosed and claimed in the aforementioned Dayapplication;

FIG. 2 is a section taken on line 22 of FIG. 1 (but notto the samescale) showing the interrelationship between the pool, the weirconstruction and the liquid coolant conducting means of this invention;

FIG. 3 is a section taken on line 34-3 of FIG. 2 and FIG. 4 isa sectionshowing the application of the instant invention to dovetailed bucketconstruction.

DESCRIPTION OF THE PREFERRED EMBODIMENT Turbine bucket 10 consists ofmetal skin 11 bonded to hollow core 12 having spanwise extending grooves13a formed in the surfaces thereof. The rectangular cooling channels, orpassages, 13 defined by skin 11 and grooves 13a conduct cooling liquidtherethrough beneath skin 11. At the upper ends thereof the rectan-.gular cooling channels 13 on the pressure side of bucket 10 are in flowcommunication with, and terminate at, manifold 14 recessed into core 12.On the suction side of bucket 10 the rectangular cooling channels thegas side heat transfer coefficient would be highly desirable. 7

Reference is-also made herein to power augmenter construction describedand claimed in U. S. Pat. application Ser. No. 285,631 Day, filed Sept.1, 1972 and assigned to the assignee of the instant invention.

SUMMARY or THE INVENTION The improved system of this invention for theintroduction of liquid coolant into opencircuit liquid cooled turbinebuckets is a very substantial answer to the need set forth hereinabove.In addition to satisfying this need the instant invention reduces theamount of coolant flow required, eliminates the flow of vapor inthesystem in a direction opposing thejcoolant flow,

provides greater selectivity of bucket: operating tem- 'a pool ofcoolant maintained adjacent the weir construction and means fordirecting the incoming liquid flow to the pool. With this constructionthe How of liquid coolant passing over the'weir construction at variousstations therealong may be roughly correlated with the given demands forcooling capacity in those coolant passages serving particular portionsof the turbine bucket.

BRIEF DESCRIPTION OF THE DRAWING The exact nature of this invention aswell as objects and advantages thereof will be readily apparent fromconsideration of the following specification relating to the annexeddrawings in which:

13 are in flow communication with and terminate at a similar manifold(not shown) recessed into corel2; near the trailing edge of bucket 10, acrossover conduit connects the manifold on the suction side withmanifold 14 via opening 15. i

The open-circuit coolant discharge from manifold 14 (and, thereby, fromthe manifold on the suction side) is accomplished viaconvergent-divergent nozzle 16 described in the aforementioned Dayapplication. Annular collection slot 17 formed in casing 18 receives theliquid component of the coolantfiowdischarge from power augmenter nozzle16 e.;g. for recirculation thereof. i

The root end of core 11 consists of a number of finger-like projections,or tines, 19 ,of varying length. These projections 19 may presentagenerally rectangular profile as shown or each tine maybe tapered towardthe distal-end thereof to present a generally triangular profile. Rim 21of turbine disk 22 has grooves 23 ma- 'chined thereinv extending tovariousdepths separated by ribs 24, the depths and widths of grooves 23matching the different lengths and widths of bucket tines 19 such thattines 19 will fit snugly therein in an interlocking relationship.

Once the proper fit has been obtained, the appropriate amount of brazingalloy is placed in each groove 23 and the buckets are inserted and heldin fixed position by a fixture. The fixture is biased to maintain atight fit between tines 19 and grooves 23 regardless of thermalexpansion. Conventional brazing alloys having melting points rangingfrom 700 to 1,100" C may be used.

Thereafter, the assembly (rim with all the buckets properly located) isfurnace-brazed to provide an integral structure.

Steel alloys may be used for the skin and core, preferably thosecontaining at least 12 percent by weight of chromium for corrosionresistance and heat treatable to achieve high strength. i

The cutting of grooves 23 into rim 21 not only provides the requisiteconfiguration for fastening the bucket root and lessens the weight ofthe rim, but in addition, the ribs 24 between grooves 23 provide areason the upper surface thereof for attachment thereto of the investmentcast platform elements 26, which have concave undersides to accommodatepools 27 of liquid coolant. Support segments 28 are dimensioned andlocated so that the widths thereof coincide with the widths of ribs 24,when placed in juxtaposition.

The weir structures 29 for metering of the liquid coolant are formedalong walls, which extend along the sides of pools 27 and are accuratelyground to some preselected radius, e.g..the radius of the initial radiusof the ribs 24. Thus, each weir 29 provides a cylindrical surface (theelements of which extend in the axial direction) following the trace ofbucket (FIG. 3) on each side thereof separating a pool 27 from adjacentcooling channels 13.

Platform elements 26 are affixed to the rotor rim 21 by the electronbeam welding of platform edges 26a and supports 28 to ribs 24 afterpreviously grinding the radially inwardly faces of edges 26a andsupports 28 to a radius common to the initial radius of ribs 24.

If the radius of those portions of ribs 24 located under platformelements 26, but not actually affixed to supports 28, remains unchanged,they can interfere with the free movement of liquid coolant in pools 27.Thus, to avoid disturbing the pool surfaces, the ribs 24 are cut back asshown in FIG. 2 in comparing are 30 (the initial outer surface of rims24) and arc 30a (the cut-back portions of rims 24).

Although the platform construction shown herein consists of individualplatform elements other constructions are equally feasible. For example,the platform components may be made integral with each bucket. In eachcase the weir surfaces distributing coolant to buckets 10 will be formedas part of the platform construction located adjacent each side of eachbucket 10.

As is described in the aforementioned Kydd patents, cooling liquid(usually water) is sprayed at low pressure in a generally radiallyoutward direction from nozzles (not shown, but preferably located oneach side of disk 22) and impinges on disk 22. The coolant thereuponmoves into gutters 32, 32a defined in part by downwardly extending lipportions 33, 33a. The cooling liquid is retained in the gutters untilthis liquid has accelerated to the prevailing disk rim velocity.

After the cooling liquid in gutters 32, 32a has been so accelerated,this liquid continually drains from gutters'32, 32a passing radiallyoutward via pool supply tubes 34, each pool 27 (one for each platformelement 26) receiving liquid coolant from at least two tubes 34.

. An alternate arrangement (not shown) for conducting liquid coolantfromgutters 32, 32a to pools 27 would consist of providing internalpassageways, which run in the general radial direction, through theouter walls of rim 21 and platform edges 26a, each passageway being inflow communication with a gutter and a pool. Entry to the pool would bemade radially outward of the weir surface 29.

As the coolant in each pool 27 passes over the surfaces of the platformelements 26, these elements are kept cool. Thereafter, the coolantpasses over weirs 29 and then into the radially inner end of coolingchannels 13 (via grooves 130) in adjacent buckets l0.

As the cooling liquid moves radially outward through cooling channels 13of any given bucket 10, then depending upon (a) the rate at whichcoolant is supplied, (b) the desired operating temperature of thebucket, (c) the area of the throat of nozzle 16 and (d) the extent offrictional heating of the coolant within the distribution circuit, someportion of the liquid coolant is converted to the gaseous or vapor stateas it absorbs heat from the skin 11 and core 12 of the bucket. At theouter ends of cooling channels 13 the vapor or gas and any remainingliquid coolant pass into manifold 14 and the manifold on the suctionface. Thereafter, the flow from the suction face manifold is merged withthe flow in manifold 14 (via opening 15) and these combined flows exittherefrom through power augmenter nozzle 16. As is described in theaforementioned Day application, incorporated herein by reference, thisexit flow is relied upon for the recovery of reaction energy and isparticularly effective when the exit flow is made supersonic.

Investigations have shown that the characteristics of flow in pools 27and the'condition of the surface of each of these pools stronglyinfluence the accuracy with which metering of the liquid coolant flow togrooves 13a may be accomplished. The pool flow characteristics and poolsurface condition are, of course, strongly affected by the rate of entryof liquid coolant into the pool, the direction and velocity of theentering liquid and the manner of entry thereof.

Thus, the flow of liquid entering pool 27 should distribute itselftherein at a uniform constant low velocity so as not to disturb thesurface 36 of pool 27 and performance of the metering function is foundto be greatly improved by introducing the liquid coolant beneath thepool surface 36. This entry is accomplished by locating the dischargeend 37 of each supply tube 34 radially outward of surface 36, end 37being spaced away from the underside of platform element 26 by means oftip 38 e.g. a projecting portion of the wall of tube 34 in contact withthe base of the concavity containing pool 27. Since the distance fromthe underside of platform 26 to surface 36 must exceed the distance fromthe underside of platform 26 to the weir surface 29 by a finite amount(e.g. about 000075 inch) in order for liquid flow to pass over weir 29,end 37 may be radially disposed at the same level as the surface of weir29 or'at some optimum position radially outward therefrom.

By introducing liquid coolant via supply tube 34 located in the mannershown, this entering liquid does not splash into pool 27, but enterssmoothly without disturbing surface 36. Any tendency of the liquidleaving gutters 32, 32a to form waves or to splash on its way to pool 27is confined to the inside of the inlet tubes 34. In fact it is ofadvantage to provide irregularities over the inner surface of each tube34-(e.g. a threaded surface) to offset the effects of radialacceleration tending to dispose the liquid coolant over the trailingwall of tube 34 as it passes to pool 27.

The promotion of smooth flow in the liquid moving laterally fromdischarge end 37 toward the central region of pool 27 is accomplishedboth by properly directing the entering liquid and by minimizingobstruction to this lateral flow. These improved conditions areaccomplished by proper selection of the size and shape of inlet tubetips 38, by locating each tip 38 around the central axis of tube 34 inan optimum position, and by employing the minimum number of support ribs28, each rib 28 being of minimum length. These support ribs 28 provideinterconnection between each platform element 26 and ribs 24 alignedtherewith.

Instead of employing separate support projections 28 to align withcertain ribs 24, each platform element 26 may be made with a singlesupport projection formed on the underside thereof extending generallytransverse of ribs 24 for attachment thereto. By proper selection of theshape of this projection an optimum pool geometry for weir flowdistribution can be achieved.

The disposition of tips 38 (illustrated in FIG. 3) around the centralaxes of the tubes 34 to which they are connected is such as to directthe entering liquid so as to cause preferential flow in a controlledmanner, i.e. at certain weir stations. Thus, in the placement shown thelateral flow of liquid entering at low velocity is encouraged to providethe desired pool depth profile over the extent of pool 27. With thecombination shown, flow at specific weir stations e.g. adjacent thepressure side leading edge and adjacent the pressure side trailing edgeof buckets is increased to compensate for the larger cooling capacityrequired over these specificturbine bucket surface areas. l

Preferably supply tubes 34 are located with the axes thereof extendingin a radial direction so that the initial velocity of the incomingcoolant flow is perpendicular to the bottom of the pool. However, ifadditional directional effect is required, it may be obtained byintroducing a slight (as much as about inclination of the axes of tubes34 to theradial direction. Tip support 34 need not be made as a singlepiece as shown, but may, if desired, be in the form of a plurality ofspaced tip extensions.

The power required to pump the coolant through the cooling circuit maybe reduced in part by reducing the mass flow of liquid coolant employed.This in turn results in the vaporization of some or all of the liquidcoolant. Generally, the volume of the vapor so generated is orders ofmagnitude larger than the volume of the liquid actually in the coolingchannels and the vapor velocities in the cooling channels is very high.Unfortunately, these vapor flows can deleteriously influence the coolantliquid flows. In particular, any flow of vapor in a direction oppositeto the direction of distribution of the liquid flow will slow down therequired transit of the liquid coolant. If this occurs, the thickness ofthe liquid film-in the coolant passages is increased and the highconvective heat transfer coefficient to the buckets is reduced. Anotheradvantage of the instant invention in addition to the increasedeffectiveness of metering of the coolant flow is that by disposing eachsupply tube 34 with the end 37 thereof below the surface 36 of pool 27,flow of vapor in the upstream direction in the cooling circuit isprevented, while still preserving the downstream flow of coolant liquid.

The aforementioned vaporization of coolant increases the pressure incoolant passages 13 and this in turn increasesthe boiling point of theliquid coolant. The pressure in coolant passages 13 will rise in thismanner until a balance is created between theproduction of coolant vaporand the escape of coolant vapor. Any upstream escape of the coolantvapor is prevented by the sealing off of the ends 37 of supply tubes 34by the. liquid in pools 27 and the increase in pressure in coolantpassages 13 becomes balanced by a rise in the to the gutters 32, 32a,the transfer of heat to buckets l0 and the area of the throat of eachnozzle 16, a condition of self-regulation is imposed whereby thedistance H is maintained. The balance of the volume of each tube 34(radially inward of the liquid level) consists essentially of air, hotgas and/or steam. Liquid coolant entering inlet tubes 34 in the rotatingsystem disposes itself over the trailing wall of the tubes 34 as ittraverses volume 39, except for portions of the flow, which may tend tosplash away from the walls within the confines of tube 34, particularlyif the inside surface thereof is roughened, until the coolant liquidreaches the liquid level.

The increase of pressure in the coolant circuit upstream of nozzle 16can be made high enough so that the discharge of vapor through nozzle 16is at supersonic velocity providing a very substantial force vector in adirection opposite to the direction of rotation of blades 10 effectiveto recoup at least in part thecoolant pumping losses.

of this invention to a dovetailed bucket made up of metal skin 51 andplatform element 52 affixed to bucket core 53 having a dovetail rootportion 54 integral therewith. i

Liquid is supplied from a coolant source (not shown) to gutters 56, 56avia passages 57, 57a extending through rotor 58. Gutters 56, 56a are inflow communication with pools 59, 59a of liquid coolant via liquidcoolant conducting tubes 61 having discharge ends 62 andpositioning/deflection tips 62. Weirs 64, 64a are formed integral withplatform elements 52, 52a. With the construction shown each bucket 50 isisolated from the rest of the turbine buckets and, furthermore, ifdesired, each side (pressure or suction side) can be isolated insofar asthe coolant distribution function is concerned. Otherwise, operation ofthe coolant distribution system is the same as has been describedhereinabove in that liquid coolant distributed over weir surfaces 64,64a enters slots 66a (grooves in the airfoil surfaces) of bucket 53 forentry into coolant passages 66 defined by skin 51 and slots 66a. Thearrangement of cooling channels 66, the collecting manifolds (not shown)and the ejection nozzle (not shown) would be the same as, or similar tothe arrangement shown in FIG. 1.

Although use of the structures shown herein need not be limited in thefollowing way, the grooved rim rotor with mating bucket root shown inFIGS. 1-3 appear to be more apt to be employed in the construction ofsmall liquid cooled turbines while the dovetailed bucket constructionshown in FIG. 4 appear to be more adaptable for use in larger liquidcooled turbine units.

This invention has been illustrated in connection with a liquid cooledgas turbine, but application, can be made thereof to any liquid cooledrotor system, e.g. a compressor, which would in essence comprise thegeneral structure shown herein operated in reverse to work on a gasinstead of having gaseous working fluid work on the rotor disk via thebuckets.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is v Y 1. In a gas turbine wherein a turbine disk is mounted on ashaft rotatably supported in a casing, said turbine disk extendingsubstantially perpendicular to the axis of said shaft and having turbinebuckets affixed to the outer rim thereof with platform structuredisposed therebetween, said buckets receiving a driving force from hotmotive fluid moving in a direction generally parallel to said axis ofsaid shaft and the driving force being transmitted to said shaft viarotation of said turbine disk, means located radially inward of saidplatform structure for introducing liquid coolant within said turbine ina radially outward direction into openended coolant distributioncircuits, each open-ended coolant distribution circuit comprisingcooling channels extending beneath the airfoil surfaces of and in agenerally radial direction along each of said buckets, metering meanslocated beneath said platform structure in flow communication with saidcooling channels and a manifold and discharge portion located in the.tip region of each of said buckets in flow communication with theradially outer ends of cooling channels of the given bucket wherebycoolant passes to the underside of said platform structure, is meteredinto and proceeds through cooling channels and exits therefrom into saidmanifold and discharge portion, 7 the improvement comprising:

a. means located in each of said open-ended coolant distributioncircuits for conducting coolant into a concavity formed in the undersideof each of said platform structures,

b. at, least one metering surface formed along wall structure definingeach concavity, said conducting means being located with the dischargeend thereof disposed at a radial distance from said axis of said shaftat least as great as the radial distance of said at least one meteringsurface from said axis of said shaft. t

2. The improvement of claim 1 wherein each conducting means is a tubehaving at least one protrusion at the discharge end thereof, said atleast one protrusion being in contact with the base of the concavity inthe underside of the platform structure.

3. The improvement of claim 1 wherein each conducting means places anannular gutter in flow communication with the concavity in the undersideof each platform structure, said annulargu'tter being located so as toreceive liquid coolant from the means for introducing liquid coolant.

4. The improvement of claim 1 wherein a poweraugmenting nozzle comprisesthe discharge portion of each open-ended coolant distribution circuit.

5. In a liquid cooled rotor system wherein a rotor disk is mounted on ashaft rotatably supported in a casing, said rotor disk extendingsubstantially perpendicular to the axis of said shaft and having bucketsaffixed to the outer rim thereof with platform structure disposedtherebetween, means located radially inward of said platform structurefor introducing liquid coolant within said rotor system in a radiallyoutward direction into open-ended coolant distribution circuits, eachopenended coolant distribution circuit comprising subsurface coolingchannels extending generally radially of each of said buckets andmetering means located beneath said platform structure in flowcommunication with said cooling channels whereby coolant passes to theunderside of said platform structure, is metering into and proceedsthrough said cooling channels and discharges from said cooling channels,the improvement comprising:

a. means located in each of said open-ended coolant distributioncircuits for conducting coolant into a concavity formed in the undersideof each of said platform structures,

b. at least one metering surface formed along wall structuredefiningeach concavity, said conducting means being located with the dischargeend thereof disposed at a radial distance from said axis of said shaftat least as great as the radial distance of said at least one meteringsurface from said'axis of said shaft.

6. The improvement of claim S'Wherein each conducting means is a tubehaving at least one protrusion at the discharge end thereof, said atleast one protrusion being in contact with the base of the concavity inthe underside of the platform structure.

7. The improvement of claim 5 wherein a poweraugmenting nozzle formspart of each open-ended coolant distribution circuit and is in flowcommunication with the discharge ends of cooling channels in saidcircuit.

8. The improvement of claim 5 wherein each conducting means places anannular gutter in flow communication with the concavity in the undersideof each platform structure, said annular gutter being located so as toreceive liquid coolant from the means for introducing liquid coolant. I

1. In a gas turbine wherein a turbine disk is mounted on a shaftrotatably supported in a casing, said turbine disk extendingsubstantially perpendicular to the axis of said shaft and having turbinebuckets affixed to the outer rim thereof with platform structuredisposed therebetween, said buckets receiving a driving force from hotmotive fluid moving in a direction generally parallel to said axis ofsaid shaft and the driving force being transmitted to said shaft viarotation of said turbine disk, means located radially inward of saidplatform structure for introducing liquid coolant within said turbine ina radially outward direction into open-ended coolant distributioncircuits, each open-ended coolant distribution circuit comprisingcooling channels extending beneath the airfoil surfaces of and in agenerally radial direction along each of said buckets, metering meanslocated beneath said platform structure in flow communication with saidcooling channels and a manifold and discharge portion located in the tipregion of each of said buckets in flow communication with the radiallyouter ends of cooling channels of the given bucket whereby coolantpasses to the underside of said platform structure, is metered into andproceeds through cooling channels and exits therefrom into said manifoldand discharge portion, the improvement comprising: a. means located ineach of said open-ended coolant distribution circuits for conductingcoolant into a concavity formed in the underside of each of saidplatform structures, b. at least one metering surface formed along wallstructure defining each concavity, said conducting means being locatedwith the discharge end thereof disposed at a radial distance from saidaxis of said shaft at least as great as the radial distance of said atleast one metering surface from said axis of said shaft.
 2. Theimprovement of claim 1 wherein each conducting means is a tube having atleast one protrusion at the discharge end thereof, said at least oneprotrusion being in contact with the base of the concavity in theunderside of the platform structure.
 3. The improvement of claim 1wherein each conducting means places an annular gutter in flowcommunication with the concavity in the underside of each platformstructure, said annular gutter being located so as to receive liquidcoolant from the means for introducing liquid coolant.
 4. Theimprovement of claim 1 wherein a power-augmenting nozzle comprises thedischarge portion of each open-ended coolant distribution circuit.
 5. INa liquid cooled rotor system wherein a rotor disk is mounted on a shaftrotatably supported in a casing, said rotor disk extending substantiallyperpendicular to the axis of said shaft and having buckets affixed tothe outer rim thereof with platform structure disposed therebetween,means located radially inward of said platform structure for introducingliquid coolant within said rotor system in a radially outward directioninto open-ended coolant distribution circuits, each open-ended coolantdistribution circuit comprising subsurface cooling channels extendinggenerally radially of each of said buckets and metering means locatedbeneath said platform structure in flow communication with said coolingchannels whereby coolant passes to the underside of said platformstructure, is metering into and proceeds through said cooling channelsand discharges from said cooling channels, the improvement comprising:a. means located in each of said open-ended coolant distributioncircuits for conducting coolant into a concavity formed in the undersideof each of said platform structures, b. at least one metering surfaceformed along wall structure defining each concavity, said conductingmeans being located with the discharge end thereof disposed at a radialdistance from said axis of said shaft at least as great as the radialdistance of said at least one metering surface from said axis of saidshaft.
 6. The improvement of claim 5 wherein each conducting means is atube having at least one protrusion at the discharge end thereof, saidat least one protrusion being in contact with the base of the concavityin the underside of the platform structure.
 7. The improvement of claim5 wherein a power-augmenting nozzle forms part of each open-endedcoolant distribution circuit and is in flow communication with thedischarge ends of cooling channels in said circuit.
 8. The improvementof claim 5 wherein each conducting means places an annular gutter inflow communication with the concavity in the underside of each platformstructure, said annular gutter being located so as to receive liquidcoolant from the means for introducing liquid coolant.